Description is provided hereinafter of a conventional transmitter receiver unit with reference to FIG. 10 and FIG. 11.
FIG. 10 is a circuit diagram of the conventional transmitter receiver unit. In FIG. 10, conventional transmitter receiver unit 1 comprises antenna terminal 2, transistor 3 and filter multiplexer 4. Transistor 3 is connected with antenna terminal 2, and it functions as a switch for selecting a communication system from among a plurality of them. Filter multiplexer 4 is connected to transistor 3, and it has receiving terminal 5 and transmitting terminal 6. Receiving terminal 5 is connected to filter multiplexer 4, while also branched and connected with a receiver circuit (not shown in the figure), which receives a signal of a first frequency band from filter multiplexer 4. Transmitting terminal 6 is connected to filter multiplexer 4, while also branched and connected with a transmitter circuit (not shown), which gives an input signal of a second frequency band to filter multiplexer 4.
FIG. 11 is a schematic illustration showing a relation of frequencies covering individual communication bands used for the conventional transmitter receiver unit. In FIG. 11, the first frequency band used as a receiving frequency and the second frequency band used as a transmitting frequency, for example, are 881.5 MHz-band and 836.5 MHz-band respectively. This structure, in particular, is used for communication systems that carry out the functions of transmission and reception simultaneously such as CDMA (“Code Division Multiple Access”). Japanese Patent Unexamined Publication, No. 2002-9679 is one example of such prior art documents known to be relevant to the invention of this patent application.
Due to the distortion characteristic of transistor 3, however, the signal of the second frequency band (836.5 MHz) input to transistor 3 from transmitting terminal 6 through filter multiplexer 4 is mixed with a signal of a third frequency band (e.g., 791.5 MHz) contained in the signal input to transistor 3 from antenna terminal 2, thereby causing a distortion of the signal of the first frequency band (881.5 MHz), or the receiving frequency band. Here, the third frequency band is a frequency band produced by addition or subtraction between integer multiples of the second frequency band and the first frequency band. Consequently, the distorted signal of the first frequency band containing information of other systems as well as information of the transmission signal passes through filter multiplexer 4, which is then input to the receiver circuit through receiving terminal 5, and it becomes an interfering signal in the receiver circuit.